An article may be cut by many methods, each selected according to known criteria including an expected use of the cut article. For example, in the gem stone industry, chunks of natural crystals are cut by splitting off portions along planes of known weakness, usually cleavage planes. The cut surfaces (called facets) along such planes are generally mirror-like and smooth with special optical properties. Light incident to the facets is predominantly reflected in a brilliant display and usually very little light passes through the facets and into the crystal.
Generally, in the semiconductor industry, crystals similar to natural crystals have been grown in a bulk, monocrystalline manner and formed into substantially planar wafers for device manufacture. The devices were cut from a wafer by rough methods such as by scribing and breaking or by sawing, probably because the cut surfaces were of little concern in operation of the devices. Such concern is considerably different in the operation of laser chips.
Laser chips often have a bulk crystalline substrate and four layers of different composition grown thereupon by epitaxial methods in an arrangement called a heterostructure. The exposed, fourth layer and the substrate facilitate ohmic contact and the substrate also provides physical strength for the chip. The first and third layers inject electrons and holes, respectively, into the second, active layer under a properly biased condition. Within the active layer electrons and holes spontaneously combine and emit photons of light. When a photon impinges upon an uncombined electron, another photon is emitted (by stimulation) having the same frequency, direction and phase as the impinging photon. Amplificaton of light results by such stimulation, forming laser light which is desirably intense and coherent. The problem is to confine the light generation process in such a manner that a desired laser beam may be obtained from the chip.
Typically, the active layer is delineated by proton bombardment to form a channel region in the chip wherein light is directed toward two opposing ends transverse to known crystalline planes. The object is to have the spontaneously emitted light be predominantely reflected by crystal facet portions (called windows) at each side of the chip where the channel region is exposed. The reflected light then stimulates free electrons to emit coherent light and a laser beam is developed which penetrates the mirror-like windows in a manner suitable for light transmission. A problem is to properly divide the chips from a wafer, but such problem is greatly alleviated by thinning the substrate portion and breaking along the desired crystalline planes. A prior problem is to cut portions from a heterostructural wafer for inspection before the chips are divided, when the substrate is still thick.
Heretofore, such cutting was performed by operator-dependent, manual methods. A typical wafer has a rectangular outline about 3/4 inch wide by about 1 inch long, the short sides being cut along a desired crystalline plane. Such cut wafer was placed upon a yielding surface and held in place with a cotton swab. Then a very sharp scriber in an almost vertical position was utilized to make short cutting strokes perpendicular and adjacent to a long side and ostensibly along the desired splitting plane. The strokes were repeated until splitting was initiated and a cut was self-propagated, hopefully along the desired plane and through the wafer. The above method was time consuming and fraught with waste caused by errant splits and scratches. Other methods utilizing conventional tools and impact forces were also too uncontrolled and low yielding to be acceptable.
Accordingly, it was desirable to develop new and improved expedients for cutting articles along planes known to facilitate splitting. Such splitting should be initiated by contact with as little of the cut surface as practical to avoid damage to such surfaces. Thereafter, the split should self-propagate along a desired plane through the article. The expedients should be particularly applicable to such synthetic crystalline articles as semiconductor wafers including those wafers having a heterostructural arrangement. It is desirable that narrow, elongate portions be cut from such wafers for inspection after heterostructural formation and before removal of unwanted substrate thickness. Moreover, the cutting should be done in a controlled, repetitive manner with a high yield of undamaged cut portions and residue portions.